The present invention relates to an improvement in a diaphragm actuator which may preferably be used in an exhaust by-pass controlling apparatus of a turbocharged internal combustion engine.
In an internal combustion engine which includes a turbocharger and a suction throttle valve upstream of the compressor of the turbocharger, suction air quantity of the engine is accurately controlled by means of a controlling apparatus including a diaphragm actuator which controls an exhaust by-pass valve to maintain the pressure difference across the compressor of the turbocharger within a predetermined value.
Such an exhaust by-pass valve controlling apparatus is shown in FIG. 1. A turbocharger 1 includes a compressor 2 which sucks air through a throttle valve 3 in a carburetor 4. An exhaust conduit which supplies the exhaust gas to a turbine 5 of the turbocharger 1 includes a by-pass passage 6 which by-passes the turbine 5. An exhaust by-pass valve 7 controls the flow rate in the by-pass passage 6 to regulate supercharged intake pressure supplied to the engine 8. The by-pass valve 7 is actuated by a diaphragm actuator 9 based on suction conduit pressure upstream of the compressor 2, which is a negative pressure, and suction conduit pressure downstream of the compressor 2, which is a negative or positive pressure.
The diaphragm actuator 9 comprises a pressure chamber 9' which communicates with the downstream side suction conduit of the compressor 2, a negative pressure chamber 10 which communicates with the upstream side suction conduit of the compressor 2, and a diaphragm 11 which defines the pressure chamber 9' and the negative pressure chamber 10. An operating shaft 12 passes through the negative pressure chamber 10 and connects the diaphragm 11 with the exhaust by-pass valve 7. A spring 13 in the negative pressure chamber 10 urges the diaphragm 11 toward the pressure chamber 9' to close the exhaust by-pass valve 7, when the pressure difference between the pressure chamber 9' and the negative pressure chamber 10 is below a predetermined value.
When the opening of the throttle 3 is small and the engine 8 is operating at a low speed range, the pressure in the upstream side suction conduit is below a predetermined value. Thus the pressure difference between the pressure chamber 9' and the negative pressure chamber 10 of the diaphragm actuator 9 urges the diaphragm 11 rightwards against the urging force of the spring 13 so that the operating shaft 12 moves rightwards to open the throttle by-pass valve 7. Consequently, exhaust gas in the exhaust manifold 14 of the engine 8 flows through the by-pass valve 7 and the by-pass passage 6 to the exhaust conduit 15 by-passing the turbine 5. By this, increase of back pressure caused by slowly rotating turbine 5 is eliminated so that the output loss is decreased.
When the engine is operated at an intermediate or high speed range, the throttle valve opening is increased so that the chamber 10 is supplied with negative pressure which is near the atmospheric pressure, and the chamber 9' is supplied with supercharging positive pressure. Thus, in the diaphragm actuator 9, the diaphragm 11 is maintained by urging force of the spring 13 in that position in which the exhaust by-pass valve 7 is closed. Exhaust gas through the exhaust manifold 14 passes through the turbine 5 to the exhaust conduit 15, and drives the turbocharger 1 to perform sufficient supercharging operation.
When the load of the engine 8 is further increased and exhaust energy supplied to the turbine 5 is increased, the supercharging pressure rises correspondingly. As the supercharging pressure is applied to the pressure chamber 9', when the supercharging pressure exceeds a predetermined value, the diaphragm 11 of the diaphragm actuator 9 is urged rightwards against the urging force of the spring 13 so that the operating shaft 12 opens the exhaust by-pass valve 7. Thus, exhaust gas energy supplied to the turbine 5 is decreased to lower the supercharging pressure. By this, an excessive supercharging is eliminated.
As shown more clearly in FIG. 2, in the above-mentioned diaphragm actuator 9, the operating shaft 12 is passed slidably through a wall portion 17 of the negative pressure chamber 10, and shaft seal means 16 prevents leakage of air from atmosphere into the negative pressure chamber 10 along the surface of the shaft 12. The shaft seal means 16 must satisfy two contrary conditions, i.e. to perfectly prevent leakage of the ambient air and to permit a smooth sliding movement of the shaft 12.
Conventionally a washer type shaft seal 16 is inserted between the wall portion 17 of the actuator 9 and a structural member 18 which is to mount the diaphragm actuator 9 in position, to provide a very small clearance between the seal means 16 and the shaft 12.
As there is a clearance between the operating shaft 12 and the shaft seal means 16, the negative pressure chamber 10 cannot be completely closed, so that the operating accuracy of the diaphragm actuator 9 is distrubed. Especially, when a very high negative pressure is applied into the negative pressure chamber 10 to actuate the diaphragm 11, leakage of ambient air through the shaft seal means 16 varies the pressure in the negative pressure chamber 10 markedly, so that the diaphragm actuator 9 is operated inaccurately.
Further, when the engine 8 is operated at a low speed and low load condition, inlet pressure of the compressor 2, i.e. the pressure in the negative pressure chamber 10, is a very high negative pressure, and the suction air quantity of the engine 8 is very small. Thus, as the ambient air leaks into the negative pressure chamber 10 through the clearance between the shaft seal means 16 and the operating shaft 12, the air which did not pass through the throttle valve 3 is additionally supplied to the engine 8. The engine 8 is normally supplied with fuel which corresponds to the air supply quantity through the throttle valve 3. Thus, the additional air which did not pass through the throttle valve 3 varies the predetermined air fuel ratio in the engine 8 so that the operation of the engine 8 becomes unstable.